We have discovered that scattered light intensity fluctuations (SLIF) are present in isolated cardiac muscle even under conditions formerly considered to be quiescent. Subsequent experiments indicated that SLIF is highly dependent on Ca++ loading of the cell and could be reversibly terminated (1) by maintaining constant (Ca++) in the myofilament space in skinned fibers or (2) in intact fibers by caffeine. These results were interpreted to indicate that cellular myoplasmic (Ca++) oscillates in diastole, producing motion of the myofilaments, which modulates the laser beam resulting in SLIF. In addition to producing SLIF, this myofilament motion which is asynchronous within a cell, and among cells, results in a small degree of diastolic force or "tone" in the muscle. In aditional experiments we have examined the relationship of the SLIF frequency and force development in response to subsequent excitation and have found very high correlations. Thus, the oscillatory Ca++ pool monitored by SLIF appears to be intimately related to excitation-contraction coupling in muscle.